1. Field of the Invention
This invention is in the field of methods and apparatus for the application of coatings to glass containers. More particularly, the present invention is in the field of methods and devices for the application of coatings of varying thicknesses to bottles, jars and the like, where the distance between the closure region and the shoulder of the container is minimal.
2. Description of the Prior Art
The utility of glass bottles and jars has been broadened by surface coating to decrease abrasion and breakage, as taught by Carl, et al., U.S. Pat. No. 3,323,889; Gatchet, et al., U.S. Pat. No. 3,516,811; Scholes, et al., U.S. Pat. No. 3,819,404; Hofmann, et al., U.S. Pat. No. 4,431,692; Lindner, et al., U.S. Pat. Nos. 4,389,234 and 4,668,268; and others. Gatchet observed the utility of avoiding all coating on the closure region of the container, known in the art as the "finish", by maintaining the finish region out of contact with the treatment gas. Several of the prior workers in this field have recognized the existence of non-linear currents in the coating-precursor stream, including omnidirectional turbulent currents and upwardly-moving convection currents.
It is known in the art of glass manufacture that uncoated glass is generally unsuitable for handling in high-speed operations, its brittleness rendering the surface susceptible to potentially catastrophic damage. Specifically, bottles and other glass containers made in large numbers are susceptible to breakage in the course of being transferred through various manufacturing steps, or during a subsequent filling operation.
In order to minimize the problems thus encountered, a number of treatments have been applied to the containers as they are manufactured. Such treatments include, e.g., spraying with lubricant such as a wax or fatty acid, and applying reactive coatings by chemical-vapor-deposition (CVD) or spray-pyrolysis methods.
Treatment by CVD typically can involve propelling a vapor of metal-containing species onto the hot glass-container surface to produce a thin layer of metal oxide, typically stannic or titanic oxide, which anchors the waxy lubricant, added after annealing. Without the metal-oxide layer, such waxy lubricants do not adhere well to glass under the conditions encountered in a filling line.
In U.S. Pat. No. 4,668,268, assigned to the same assignee as the present invention, Lindner et al. teach the application of a metal-containing compound, generally an organotin material, to the surface of a glass container immediately after that container has acquired sufficient mechanical integrity to maintain its shape on a material-handling line. The Lindner et al. disclosure describes a coating hood for applying a uniform protective coating to a glass container as the container is transported by material-handling means, generally a conveyor belt, after its formation from molten glass. The described coating hood comprises a pair of side walls with a coating jet in at least one of the walls, and an exhaust system to remove the process stream from the coating zone.
In the field relating to the coating of containers generally and glass bottles specifically, one problem which is repeatedly encountered is that of applying adequate material to the body of the container while keeping the finish region of the container untreated. It is desirable to maintain the finish portion relatively free of coating material for both chemical and physical reasons. Where a tin compound is deposited on the screw threads of, for instance, a jar for holding baby food, the chemical, electrochemical or mechanical interaction between the metal cap and the coating may be sufficient to discolor the glass or to corrode the metal. Another disadvantage of coating on the finish is the possible effect on the frictional interaction between the glass and the cap or other closure; low friction can permit leakage, while too high a frictional value can impede both placement and removal of the closure. In either case, the utility of the treated container is adversely affected.
In the art of coating glass containers, the film deposited onto the glass surface is measured in arbitrary coating thickness units (CFU), the unit thickness being about 2.5 Angstroms (.ANG.); metal-oxide coatings of from about 30 to 40 CTU, or about 60.ANG. to 100.ANG., may be required for the body of the container, while acceptable coating on the finish may be one-half or even one-tenth of this amount, depending upon the ware and its intended use. While manufacturers of baby food state a preference for a finish-coating thickness of less than half that of the shoulder coating, proximity of finish to shoulder has heretofore made the desired separation difficult or impossible to achieve under the teachings of the prior art.
The improvement in the art which Lindner et al. provided in partial response to the problem of differential wall and finish thickness was accomplished by directing a stream of air in which no coating material was entrained onto the finish in order to displace and dilute coating material which would otherwise coat the finish region almost as much as the shoulder or the body of the container. By selection of the geometry of the coating hood as a function of the containers to be coated, Lindner et al. were able to effect acceptable coating thickness on the sidewalls, concomitant with protection of the finish for the large volume of glass containers having necks of appreciable length, such containers comprising beer, wine and soft-drink bottles and the like.
However, while the utility of the apparatus of the Lindner et al. patent is excellent for beer, wine and other bottles with relatively long necks, it is only marginal for applications such as food and cosmetic bottles and jars having very short or non-existent necks, i.e., where the finish region is juxtaposed adjacent the body. The latter group comprises, e.g., jam, jelly and cold-cream jars, and containers for baby foods, peanut butter, thixotropic salad dressings and the like. For the purposes of this specification, the terms "bottle" and "jar" may be used with substantial interchangeability, although generally, a jar has a relatively small separation of body and finish, while a bottle has a substantially larger separation.
While the most effective prior art directs the vapor-laden air stream horizontally at the label panel of the ware, and a stream of vapor-free air at the finish region, some of the coating stream has been found inevitably to be displaced toward the finish by conditions within the coating hood. Displacement can originate in, e.g., the shearing interaction between adjacent coating streams traversing the hood in opposite directions; in the turbulence caused by the ware as it crosses the coating streams; in the strong convection currents caused by hot ware moving through a coating stream that is typically hundreds of degrees cooler than the ware; and in the induced draft caused by the finish-protection stream. Further, even the best of the prior-art devices tend to require careful attention in order to maintain adjustment of the parameters required for an acceptable differential between the body or shoulder and the finish region.